Action Points

Note that this observational study suggests an association between shift-work and fecundity among women undergoing ovarian stimulation.

Overall, the magnitude of effects was quite low; it is unclear how these findings will translate into live births, for example.

New evidence bolsters the idea that jobs requiring heavy lifting or working nights may diminish a woman's ability to become pregnant.

In a study published February 7 in Occupational and Environmental Medicine, "women working non-day shifts and those who had more physically demanding jobs had fewer mature oocytes retrieved after controlled ovarian hyperstimulation," wrote Lidia Mínguez-Alarcón, PhD, and colleagues from Harvard University's schools of medicine and public health.

Study participants who reported lifting or moving heavy objects at work had 1.0 (P=0.08) and 1.4 (P=0.007) fewer total and mature oocyte counts compared with those who did not, for respective decreases of 8.8% and 14.4%. Moreover, women who undertook moderate to heavy physical exertion at work had 1.3 (P=0.02) fewer mature oocytes on average than woman with light physical exertion at work.

Similar results were seen for women working nights or on rotating shifts.

Occupational factors including shift work and higher levels of physical labor both had been previously suggested as factors in various reproductive outcomes, but none of those prior studies measured biomarkers of fecundity, defined as the biologic capacity for reproduction. (Fertility is defined as demonstrated fecundity, typically measured by live births.)

The current report evaluated 473 participants in the Environment and Reproductive Health Study, established in 2004. Information on work scheduling and physically demanding work was collected via take-home questionnaires. Women who reported moving or lifting heavy objects at work were less educated on average compared with women who reported never moving or lifting heavy objects at work (49% versus 66% with graduate school education). Women also reported whether their typical work shift happened during the day, evening, night, or on a rotating basis.

In total, 40% of women reported lifting or moving heavy objects at work, 22% reported moderate or heavy physical exertion at work and 91% worked day-only shifts.

All participants were evaluated based on ovarian total antral follicle count, which was obtained through transvaginal ultrasonography, and a measurement of follicle-stimulating hormone, which was collected on the third day of the menstrual cycle using an automated electrochemiluminescence immunoassay.

The median age and body mass index of the 473 women was 35 years (32–38) and 23.2 kg/m2 (21.2-26.2), respectively. The majority (80%) of women had undergone a previous fertility examination, though less than half of the women (48%) had been treated for infertility before the study. Unexplained infertility was the primary infertility diagnosis at enrollment (44%).

Women who reported lifting or moving heavy objects at work had a lower antral follicle count (adjusted difference=−0.7) compared with women who did not (P=0.06). Frequency of moving heavy objects was unrelated with day-three follicle-stimulating hormone levels. Typical work schedule and level of physical exertion at work were not associated with total antral follicle count and day-three follicle-stimulating hormone levels.

"These findings have important clinical implications because women with fewer oocytes, particularly mature oocytes, would have fewer eggs that are capable of developing into healthy embryos," said study co-author Audrey Gaskins, ScD, of Harvard's T.H. Chan School of Public Health, on the BMJ Talk Medicine podcast. "Our results also suggest that occupational factors such as shift work and physically demanding work may be more specifically affecting oocyte production and quality, rather than accelerating ovarian aging."

A difference also appeared based on the timing of a woman's work shift. Those who worked evening, night, or rotating shifts had 2.3 (P<0.001) fewer mature oocytes on average compared with women who only worked during the day. This association was stronger when evening and night shift workers were lumped together (3.2 fewer mature oocytes compared with women working day shifts).

The association between frequency of moving heavy objects at work and number of mature oocyte yield appeared to be stronger among overweight or obese women and older women compared with young women. However, tests for interaction were not significant (P=0.19 and 0.26, respectively).

According to Gaskins, more research is needed, but the findings as they are have real clinical implications.

"While our results need to be replicated in other cohorts before strong recommendations can be made, women who are planning pregnancy should at the very least be cognizant of these potential negative impacts," Gaskins said. "And they should be aware that two exposures that are relatively common in today's world could have a potentially damaging effect to their fecundity."

The study was supported by NIH grants from the National Institute of Environmental Health Sciences (NIEHS) and from the National Institute of Child Health and Human Development (NICHD).

None of the authors or commenters disclosed any relationships with industry.

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